Hemp lime
Hemp lime , also called hempcrete (English Hempcrete), is a natural composite material , which essentially consists of the light wood of the hemp plant ( Cannabis Sativa L.) , the shives and a calcareous binding agent. Due to the lack of aggregate, it is assigned to the agricultural concretes. Hemp lime is mainly used as a natural insulation material for walls, roofs and floors and can also be used as insulation plaster . Due to the open-pored structure of the shives and the lime, this material has a low thermal conductivity and has a hygroscopic effect . Due to the carbon fixation during the growth of the hemp plant, the building material has a negative CO 2 balance and thus functions as a CO 2 sink .
Hemp lime can be introduced into formwork and applied by spraying or manufactured in prefabrication as a stone or prefabricated component. The building material is dimensionally stable and solid, but has only low compressive strengths and can only be used for monolithic, multi-storey solid constructions in connection with a load-bearing framework .
composition
The raw materials of hemp lime are hemp shives and a lime-containing binding agent. Often used binders include natural hydraulic limes such as NHL 5, natural quick-release cements , or hydrated lime with pozzolanic aggregate (e.g. metakaolin , fly ash & blast furnace slag ). The elongated shives are the organic aggregate for the binder. Their share is about 75% of the mass. They are flat and oblong in shape. Their properties for the hemp lime depend on the one hand on the geometry (mainly the length) and on the other hand on the fiber and dust content.
Hemp lime can be produced in different proportions between binder and aggregate. The higher the proportion of binder, the higher the bulk density of the hemp lime and the lower the heat-insulating properties. Depending on the intended use, hemp lime is usually produced in bulk densities of 200 kg / m³ - 500 kg / m³. Mixtures for roof insulation have gross densities of 200–250 kg / m³, for wall and floor applications the gross density increases to 400 or 500 kg / m³. In these areas, the building material has a thermal conductivity of approx. 0.06 W / m * K - 0.12 W / m * K. Furthermore, the composition has an impact on the moisture-regulating and sound-absorbing properties and the load capacity.
In addition to the proportion of binder, the bulk density is influenced by factors such as the sheave size and the compression applied during installation. It is also decisive for the proportion of pores in hemp lime, which is approx. 70% by volume with a bulk density of 400 kg / m³. The type of pores is divided into three categories according to their size:
- Macropores (approx. 1 cm) arise due to the unstructured arrangement of the shives in the mix
- Mesopores (0.01 mm - 1 mm) are found in the sheave structure and as air pockets in the binder
- Micropores (<0.01 µm) are located between hydrates of the binder matrix
properties
The hemp shives and the binding agent interact in different ways. Depending on the composition, the mechanical properties, the hygrothermal behavior, the acoustic properties, the fire behavior and the CO 2 binding capacity are influenced. These parameters are described in general terms below.
Mechanical behavior
The mechanical properties depend on the one hand on the type of binder and its mechanical strength. On the other hand, these binder properties are shaped by the flexibility and composition and quality of the shives, since the shives influence the stiffening behavior of the binder.
The porosity with the accompanying compressibility d. H. Compressibility of the shives severely limits the maximum achievable strength of the building material and at the same time ensures an unusual ductile or elastoplastic deformation behavior. Investigations of various hemp lime formulations showed that the compressive strength of hemp lime after 28 days was always between 0.25-3 MPa (approx. 1/20 of normal concrete ), accompanied by large deformations, depending on the type and proportion of binding agent. Even if the strength of hemp lime increases due to constant carbonation , it is very low compared to other building materials. This property limits the use of hemp lime in construction and requires the combination with a load-bearing framework.
Hemp lime acts as a stiffening support on the stud frame, as it can transfer loads from unstable areas within the structure in the event of horizontal loads. This greatly delays failure of the wooden structure. The ductile behavior of hemp lime makes it particularly suitable for use in earthquake-prone areas because the point of failure of the construction is delayed by the large deformation that this material allows.
Hygrothermal behavior
Hemp lime is a hygroscopic material with strongly fluctuating pore diameters in the range from nanometers to millimeters. Because of this open pore network, the building material is able to exchange water vapor with the ambient air. At high humidity, the water vapor can condense on the surfaces of the pores to form liquid water , whereas the opposite occurs with dry air. This phase transition phenomenon (evaporation / condensation) results in the absorption or release of thermal energy, which has a direct influence on the thermal insulation properties of the material. Due to this phenomenon, hemp lime is also assigned to the phase change materials (PCM).
The thermal conductivity of the common densities for hemp lime (200 - 500 kg / m³) is between 0.06 - 0.11 W / m * K (at 20 ° C; 50% RH). The heat capacity of hemp lime is around 1000 J / kg * K.
Sound absorption level
The degree of sound absorption of hemp lime can vary widely, as it depends on various parameters such as the type of binder, the mixing ratio and the processing method. Of these parameters, the shives / binder mixture ratio has the greatest influence on sound absorption.
Fire behavior
According to the European standard ( DIN EN 13 501 ), hemp lime has the fire behavior : B-s1-d0. The hemp shives are divided into Euroclass E according to the same standard. This fire behavior can be improved by sheathing with a mineral binder. Fire resistance tests have shown that walls made of hemp lime can provide a fire resistance of F90.
CO 2 absorption
Various life cycle assessments prove the sustainability of this building material and the potential to bind CO 2 . Depending on the production scenario and composition, values of 70 kg CO 2 / m³ to 307.26 kg CO 2 / m³ are given. These values exceed the CO 2 quantities emitted for production and transport (approx. 50 kg CO 2 / m³) and give hemp lime a negative CO 2 balance.
Two different mechanisms are responsible for the CO 2 uptake of hemp lime. On the one hand, the hemp plant absorbs CO 2 from the atmosphere during photosynthesis and uses it to form plant matter . More precisely, hemp consists of around 46% carbon. This high content, combined with the rapid growth and good agricultural tolerance, make hemp an ideal storage plant.
The second phenomenon of carbon binding occurs as a result of the carbonation of the calcareous binder. When calcium carbonate reacts to calcium oxide through a burning process, in addition to the emissions from the burning process, CO 2 is also created by the decarbonization of the limestone. The latter can be bound again in certain quantities during the use phase of the hemp-lime building material through carbonation. In comparison to this, however, the CO 2 binding potential of the hemp plant is much higher.
Processing methods for solid components
Hemp lime can be implemented in a number of ways. On the one hand, there are " in-situ procedures" mixed on site, which are divided into the manual introduction of the material into formwork and the spraying using a pointed machine. On the other hand, hemp-lime bodies can be prefabricated in the form of stones or wall elements and installed on the construction site.
Manual introduction
The development of the hemp lime and its introduction process is largely based on the renovation of historic half-timbered houses in France. Therefore, the original method of using hemp lime was to fill it in a form-giving form. The mixture is produced in a compulsory mixer and must then be brought to the filling point by the processor using a wheelbarrow or bucket. The light, incoherent consistency of the moist hemp lime can neither be processed in bulk nor compacted by vibration or shaking in the formwork and must therefore be compacted by vertical pressure, by hand or using a tool. The material must be installed and compacted in layers in order to avoid voids and weak areas in the wall. To ensure adequate compaction, the filling height of the building material should not be more than 0.2 meters per layer. With compulsory mixers, up to 24 m³ of hemp lime can be produced per day. Due to the manual work, the amount that can be paved with this method is limited to around 10 m³ per day.
Spraying process
In the spraying process, hemp shives and binding agents are processed with a spraying machine specially developed for the building material. The types of process are divided into wet and dry spraying processes. With these processes, manufacturers state that they can process approx. 6 m³ per hour. A single-sided, permanent formwork is usually used as the formwork system. The stud frame is moved towards the lost formwork, as the backfilling of cavities behind it cannot be implemented with this method.
When projected horizontally, hemp lime is sprayed onto the formwork using air pressure, to which it adheres and is compacted at the same time. Several layers are applied until the desired wall thickness is reached. An even distribution of the building material and an even wall surface are achieved by pulling it off with large grapes. When spraying, not all of the hemp shives stick to the wall and fall to the floor as a rebound part, coated with binding agent. In order to use this material again, the material must be mixed with fresh hemp lime so that it is not installed too dry.
Stones
In the manufacture of stones, the hemp lime is poured into molds and compacted by vibration. In addition, the stone produced in this way can also be subjected to a constant load due to compression. In addition to manufacturer-specific shapes, the stones come in various dimensions (mostly variable in width) so that they can be used for various applications. Monolithic wall structures can be created with particularly wide stones, while medium-sized formats can be combined with other hemp-lime construction methods such as B. can be connected to spraying and the thinnest dimensions can be used for renovations indoors and outdoors.
By drying the stones at the factory, time can be saved in the construction process compared to in-situ methods. When building, the stone is placed in a thin layer of lime-sand mortar and the contact points must be moistened. In order to adapt the stones to the stud frame or to create other deviations in shape, such as curves, the stones can be cut to the desired dimensions with a saw.
Prefabricated wall elements
In order to produce wall elements, hemp lime is filled into a stud frame section at the factory and delivered to the construction site after it has dried. This is a design that is particularly useful for very large projects, such as B. warehouses, industrial halls, and large public buildings are profitable. Compared to stones, the absence of mortar joints should also be mentioned, which improves the thermal insulation of the building envelope.
application areas
Most buildings made of hemp lime worldwide are residential buildings such as single or multi-family houses. But larger, public and industrial buildings can also be realized with the building material.
The Bridge Building
The Bridge Building is a campus building of the University of Bradford. It was built in 2016 and is the largest monolithic hemp lime project that has been implemented worldwide. The building has four floors and a floor area of 1800 m². All wall elements are made of hemp lime and were sprayed. Two spray machines were used simultaneously on the construction site to speed up the construction process. In addition, a two-sided formwork was used, which was filled vertically by the spray machines. The stud frame was arranged towards the inner wall.
The building achieved a BREEAM balance of 95.2%, making it the most sustainable building in the world in the education sector. The architects from Farrell & Clark and the client made sure from the start that the building was equipped with as many passive energy sources as possible. Therefore, the extremely positive environmental balance can be attributed to the highly efficient technical building equipment and the environmentally friendly and room climate-improving properties of the hemp-lime.
Web links
- Why hemp concrete is the better building material - building materials made from hemp - Hanf Magazin
Individual evidence
- ^ A b c Allin, Steve .: Building with hemp . Seed Press, Kenmare, Co. Kerry 2005, ISBN 0-9551109-0-4 .
- ↑ a b c d e f g h i j k l Arnaud, Laurent; Amziane, S .: Bio-aggregate-based Building Materials . In: Civil engineering and geomechanics series . 1st edition. 2013, ISBN 978-1-84821-404-0 .
- ↑ a b c d Sparrow, Alex .: The hempcrete book: designing and building with hemp-lime . Cambridge, ISBN 978-0-85784-224-4 .
- ↑ Schönthaler / hemp-lime brick / building with hemp. Accessed December 28, 2019 (German).
- ↑ a b c Jami, Tarun; Karade, SR; Singh, LP: A review of the properties of hemp concrete for green building applications . Ed .: Journal of Cleaner Production. 2019, p. 17 .
- ↑ Arnaud, Laurent; Gourlay, Etienne: Experimental study of parameters influencing mechanical properties of hemp concretes . Ed .: Construction and Building Materials. 2012, p. 50-56 .
- ↑ Niyigena, César; Amziane, Sofiane; Chateauneuf, Alaa; Arnaud, Laurent: Variability of the mechanical properties of hemp concrete . Ed .: Materials Today Communications. 2016, p. 122-133 .
- ^ Material testing institute University of Stuttgart: CE Bafa New. Hemp lime insulating plaster (2018)
- ↑ Amziane, Sofiane; Collet, Florence: Bio-aggregates Based Building Materials . tape 23 . Springer Netherlands, ISBN 978-94-024-1030-3 .
- ↑ Magwood, Chris: Essential hempcrete contruction: the complete step-by-step guide . Gabriola Island, BC, Canada, ISBN 978-1-55092-613-2 .
- ↑ Hempcrete Mixer. Retrieved April 3, 2020 .
- ↑ a b Spraying hemp lime with EREASY - effective processing of hemp lime. Retrieved on April 3, 2020 (German).
- ^ Green Grow Award: The Bridge Building. Retrieved March 12, 2020 .
- ^ University of Bradford - The Bright Building. Accessed April 3, 2020 (English).